Especially for the molding of thin-walled cups and similar articles, it is essential to insure an exact alignment of the core with a coacting cavity in the female mold portion. If the core is lightly tapered, the molded article adheres to it when the mold is opened and will therefore have to be stripped off by a ring closely hugging that core. Such a stripper ring, carried on a stripper plate extending radially beyond the mold, can also be used as a means for relatively centering the core and the cavity; injection molds have, in fact, been so constructed in the past.
In order to insure accurate centering of a removable stripper ring with reference to the associated core, the mold assembly of my above-identified prior patent includes a locking ring having tapering first and second inner centering surfaces engageable with respective outer centering surfaces engageable with respective outer centering surfaces of the corresponding mold portions, the locking ring being secured to a stripper plate which is slidably supported between the platens on their tie bars (either directly or through the intermediary of one of these platens) for movement parallel to the tie bars in a mold-open position. The stripper ring is seated in the locking ring between the centering surfaces thereof so as to be entrainable by the stripper plate together with the locking ring.
As long as the locking ring and the coacting mold portions are precisely machined to given specifications, that assembly operates entirely satisfactorily. Difficulties may arise, however, in the case of dimensional deviations which may cause either or both of these mold portions to abut against the stripper ring before becoming firmly seated in the corresponding centering surface of the locking ring or, conversely, may leave clearances between the stripper ring and the end faces of the mold portions. With such an imperfect fit, coaxiality of mold core and cavity would be lost or plastic molding material could flash into the space between the cavity and the stripper ring, both these occurrences resulting in faulty workpieces.
While the stripper ring of excessive axial height could be readily machined to proper thickness for accurate centering, a stripper ring of insufficient thickness would require machining of one of the tapering inner surfaces of the locking ring which is much more difficult to accomplish. Moreover, with the usual vertex angle of these tapering surfaces, a given increase in their radius results in about a fourfold reduction in the axial height of the mold stack.
This problem is intensified in a multicavity mold where the stack height must be the same for all the cavities and where, therefore, an improper fit at one cavity--due to manufacturing tolerances or wear--would require all tapers to be reground and all axial lengths to be trued to insure accurate centering and flash-free performance.